In large sized hydrogen producing apparatuses for supplying hydrogen necessary for a refining process (a hydrodesulfurizing process, a heavy oil upgrading process or the like), a steam boiler has been additionally used in order to stably supply hydrogen. However, in a middle or small sized (1˜1000 Nm3/h) hydrogen producing apparatus such as a fuel cell system for an automobile, household, distributed electric generation or the like, in order to increase its thermal efficiency and decrease its size, it is required to supply steam through self-recovery of the heat of exhaust gas or reformed gas. In a steam reforming process, it is very important to maintain the ratio of steam to hydrocarbons constant, and when a two-phase stream occurs in a state in which water is not completely vaporized, the flow rate and composition of a product is changed by feed fluctuation, so that the operation of a system becomes entirely unstable, thereby causing the breakdown of measuring instruments. Further, when the ratio of steam to hydrocarbons (S/C) instantaneously changes, the activity of a catalyst is deteriorated by coking etc., thereby decreasing the durability of an apparatus. For this reason, a heat exchanger network or a heat exchanger which can stably supply steam through self-recovery of heat must be designed and operated.
Further, at the same time as attaining stable operation, it is required to attain high thermal efficiency without additional apparatuses or costs. In particular, in the case of a hydrogen generator for a fuel cell for electric generation, the thermal efficiency of the hydrogen generator can be increased by recovering excess heat using hot water and then converting the recovered heat into usable heat.
Furthermore, the hydrogen generator must be smoothly operated without additional problems (hydrogen-related safety verification, supply and demand of related apparatuses, power loss, etc.) occurring when main apparatuses are preheated by a commonly-used electric heater and problems with the approval or permission of a hydrogen generator by introducing nitrogen (N2) into a feed supply line to preheat main apparatuses (a water gas converter for converting carbon monoxide (CO) in reformed gas into hydrogen (H2) and carbon dioxide (CO2), and, if necessary, a desulfurization reactor) to their reaction temperatures and by recycling the reacted gas at the rear end of the water gas converter.